the purpose of this study was to identify and confirm biochemically three phases of the menstrual cycle in 14 well conditioned athletes and then to examine the effects of the various phases on work capacity, perceived exertion and lactic acid accumulation following individualized work tasks. radioimmunoassays of plasma estradiol and progesterone were used to confirm the midluteal and mid-follicular phases of the cycle. subjects performed a 12 minute sub- • maximal work task consisting of 3 minutes at 60%, 3 minutes at 70% and 6 minutes at 80% of their measured maximum work capacity. perceived exertion was recorded every 2 minutes using the borg 15 point scale. after a 15 minute rest the subjects were asked to run as long as they could at a work rate corresponding to 100% of their maximum work capacity. blood samples were drawn for lactic acid assays prior to the start of the initial run, 2 minutes after the sub-maximal run and 5 minutes after the run to exhaustion. the phase of cycle had no influence on perceived exertion or on lactic acid accumulation. maximum run times to exhaustion increased significantly (p < .05) in the midfollicular and midluteal phases when compared to day 1. the average increases were 17.9% and 40.2% respectively. these findings indicate that work capacity is maximal in the midluteal phase.